Acoustic-Waveguide Sonar Finds Enormous Fish Shoals
Overfishing has devastated the oceans' stock of fish. Backscattered
echoes of low-frequency signals may provide an accurate census of that stock.
April 2006, page 20
In
May 2001, MIT's Nicholas Makris and his then postdoctoral fellow Purnima Ratilal were searching
for the ancient riverbeds buried within the continental shelf off the New Jersey coastline. Although
hidden in muck when the sea level rose during the last ice age, those ancient channels had been previously
mapped using high-frequency sonar that distinguished the slight density differences in the sediment.
Researchers sent a signal, listened for its echo, and then deconvolved the spectral components
to resolve underwater features.
Makris wondered if low-frequency sonar
could tell whether those ancient features were also the source of acoustic clutter, anomalous
backscattering prevalent in coastal regions.1 But instead of the correlations some
hoped to see between riverbeds and clutter, he, Ratilal, and their colleagues noticed puzzling
and transient features—features that turned out to be gigantic shoals of tens of millions
of fish, some assembled into an area 10 kilometers wide.
Although fish move and riverbeds
don't, distinguishing the two in backscattered acoustic signals can be tricky. Shipping lanes
along coastlines are noisy; the presence of turbulence, eddies, and tidal fluctuations can decorrelate
acoustic modes; and the speed of sound varies with the ocean's temperature and salinity. Even small
density variations between water layers form internal waves that cause sound waves to fluctuate
(see reference 2 and the article by Bill Kuperman and Jim Lynch in PHYSICS TODAY, October 2004, page 55).
It took two years and a more
controlled experiment for Makris's team to be sure that the dynamic changes produced by fish were
not simply fixed features that, "like the glint on a spoon from a moving flashlight," as Makris puts
it, were transient because of his ship's motion. The recently published account of the experiment
details the structure and dynamics of perhaps the largest massing of animals imaged in nature.3
Highs and lows
During conventional
acoustic surveys, researchers transmit high-frequency sound pulses vertically downward from
a ship that moves slowly along narrow transects. Such surveys provide a record of the backscattered
echoes from fish in a highly localized slice of ocean perhaps tens of meters wide. That approach
has long provided local estimates of fish populations but is time-consuming and misses vast areas
that fish may inhabit.
In shallow coastal shelves, where Makris's
team worked, the ocean behaves like a waveguide that traps sound between the air and sea floor; the
huge difference in density and sound speed at those interfaces causes sound to scatter and reflect
with little attenuation. The entire water column acts like a plucked guitar string. Normal modes
of the waveguide propagate and spread cylindrically, with intensities that fall off inversely
with distance—far more slowly than the spherical spreading in conventional sonar geometries.
Moreover, audio-frequency signals, at a few hundred hertz, suf-fer far less attenuation from
absorption and scattering than the high-frequency signals of 20 to 100 kHz used in conventional
sonar.
The waveguide properties
of a coastline make it possible to send low-frequency signals and receive their scattered echoes
over a huge range—thousands of square kilometers. Figure 1 illustrates Makris's technique:
One ship, moored at sea, uses a vertical array of speakers to radially transmit a 1-second broadband
chirp. A long array of hydrophones towed behind another ship, typically less than a few kilometers
away, picks up the backscattered signals from all directions.
The challenge is to reconstruct
what's in the sea. Essentially an underwater antenna, the receiver array is long compared to the
wavelength and distinguishes which waves come from which directions by monitoring the delay in
their arrival times at different points along the array. To make sense of all the convoluted returns,
the researchers deconvolve the various ocean contributions to the original pulse. In effect,
this "match filtering" selects echoes that closely resemble the original waveform template to
maximize the signal-to-noise ratio. The recipe—basically multiplying the complex conjugate
of the Fourier transform of the transmitted pulse with the transform of the received pulse—has
the effect of taking all the signals over the bandwidth and compressing them. The pulse compression
dramatically increases the temporal resolution.
The bare bones of Makris's
imaging technique has been in the literature for decades. V. H. Lichte described the first rudimentary
ocean acoustic waveguide in 1919, and the US Navy used long-range echo sounding to search for submarines
even before World War II. David Weston first demonstrated the use of long-range active sonar along
the Bristol Channel to detect fish shoals in the early 1960s and was aware of the principal reason
that fish scatter low frequencies so well—their swim bladders resonate at audio frequencies.
J. S. M. Rusby later confirmed the fish backscattering in 1971 using a towed-array system to explore
a Scottish inshore herring fishery.4
Acoustical processing
has matured since then. Advances in modeling the way sound propagates and scatters in a fluctuating,
range-dependent waveguide and better signal processing methods are largely what separate Makris's
relatively high-resolution images of fish population density from the pioneering efforts of
Weston and Rusby. Moreover, today's computers are fast enough to sort through the vast data within
minutes and reconstruct snapshots of the ocean—even movies of how shoals change dynamically.
It takes much longer for fish to swim across one of the roughly 30-m cells that form a pixel than for
the acoustic waves, traveling at around 1500 m/s, to interact with everything in a typical wide-area
image.
Consider the sequence of
images in Figure 2. Each picture registers measured echo intensities—corrected for propagation,
intensity fluctuations, and varying sonar-resolution footprints—at a particular time
as a function of position from the source and receiver arrays. To properly interpret those scattering
intensities in terms of fish populations in areal views, Naval Research Laboratory biologist
Redwood "Woody" Nero made conventional, line-transect measurements through the shoals in a separate
vessel. Specifically, he measured the scattering strength of individual fish locally with high-frequency
acoustics. That allowed the team to translate their audible-frequency intensity readings into
population estimates.
At the moment, researchers
must tolerate some uncertainty in that translation: It's difficult to reliably distinguish species,
and scattering strength varies among species. Moreover, the resonance frequency of a swim bladder
can vary with depth. Using nets to trawl for local samples can help, although that information comes
with its own bias: Some species are far easier to catch than others. The likely suspects off the New
Jersey coast include Atlantic herring, scup, hake, and black sea bass.
Still, Makris says that
the team's population estimates are accurate to within 1 dB, and the dynamic range in the images
spans more than three orders of magnitude.
School rules
Evolution
has hardwired fish to congregate into schools, and those schools into larger shoals. The behavior
maximizes eating opportunities and minimizes the risk of being eaten; herring, for instance,
typically open their gill rakers wide and filter-feed on plankton for longer times and with less
risk while swimming in groups. A large school also presents a confusing target to predators, as
individual fish blur into the crowd. To combat a threat—a shark following closely behind
the school, say—fish resort to defensive tactics. A compact school may suddenly explode
like a hand grenade, whereby the fish dart off in all directions thanks to special nerve cells. Alternatively,
individuals may gradually peel off from the school, just outside of visual range, only to reassemble
somewhere behind the predator.5
Makris's team noticed the rapid fluctuations
in shoal populations, whether in response to predation or some other prompt, that bear out such
behavior dynamics. The numbers could vary by as much as 20% over the course of a few minutes as a few
million fish were observed to migrate between schools within the shoal or through narrow bridges
that briefly connect one region to another.
The behavior is tied to the
evolutionary preference of fish to quickly assemble and reassemble into schools, explains Tony
Pitcher of the University of British Columbia's Fishery Centre. Indeed, schooling fish are biologically
adapted to copy the movements of their neighbors. Special sensory hairs in a fish's lateral line
system, located within shallow canals just below its skin and scales, pick up transient pressure
waves from close neighbors.
Uwe Kils, while working
at the Institute for Marine Research in Kiel, Germany, realized in the late 1980s that the evolutionary
behavior goes beyond simply maintaining a school's organizational coherence. It can serve to
pass a signal through the school, an effect Kils termed synchrokinesis. As a few fish quickly turn
to avoid a predator, say, nearby ones follow suit as the pressure wave travels through the crowd.
Such fish density waves can exceed the typical speed at which fish swim by an order of magnitude,
Makris says. Monitoring the large-scale internal motion and migration patterns in shoals should
address the degree of coordination and interaction between different species that may coexist
in shoals in the wild.
Apparently, what happens
at small scales also happens at large scales. Makris's observation of synchrokinesis, structure,
and rapid reassembly dynamics at large scales bears out what biologists have observed in far smaller
schools—on a scale of tens of meters.
The preference to rapidly
form large shoals from smaller subunits makes fish populations especially vulnerable to overfishing,
cautions Pitcher. Even though overfishing has diluted the stock of fish in the world's oceans to
roughly 1% of estimated abundances 100 years ago, fishermen remain able to reach their catch quotas.
The trick lies in finding the shoals, whose individual densities may be little changed. The true
depletion in fish stock may then appear invisible even to the fishermen, as populations shrink
under their feet. Pitcher remains sanguine, arguing that what fishery regulators really want—an
accurate record of the biomass in the oceans—is within reach.
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